As charging methods of a lithium ion battery, when divided roughly, a constant-current/constant-voltage charge method and a pulse charge method are used. In the constant-current/constant-voltage charge method, it is possible to shorten charging time by increasing the charging current for a lithium ion battery and by making the constant voltage applied to the lithium ion battery in charging a little higher than the full charge voltage of the battery. However, when the lithium ion battery is overcharged, there is a possibility that the performance of the battery is degraded. On the other hand, the pulse charge method causes little damage to the battery since an idle period is taken during the charging of the lithium ion battery.
As such pulse charge methods, there are three methods as follows.
As disclosed in Japanese Laid-Open Patent Application No. 6-113474, there is a first method that completes charging when the voltage in the idle period reaches a predetermined voltage.
There is a second method that makes conditions for starting the charging and suspending the charging, and repeats the charging and suspension of the charging under the conditions. The charging is completed when the charging suspension period lasts equal to or more than a predetermined time, or when the ratio of the charging period to the charging suspension period exceeds a predetermined value. For example, the charging is suspended when the voltage of the battery reaches a first voltage and the charging is restarted when the voltage falls to a second voltage during the charging.
As disclosed in Japanese Laid-Open Patent Application No. 7-336908, there is a third method that alternately repeats the charging at a high level voltage and a low level voltage and completes the charging when the charging current at the low level voltage is equal to or less than a predetermined current value.
However, in the above-described first method, there is a problem in that the charging time becomes longer compared with the constant-current/constant-voltage method. In addition, in the above-described second method, the charging time is shortened to some degree compared with the constant-current/constant-voltage method. However, since each of the charging period and the charging suspension period varies drastically between the start of the charging and just before the end of the charging, the frequency of switching the charging period and the charging suspension period varies over a wide range. Thus, there is a problem in that noise occurs over a wide frequency band.
Additionally, in the above-described third method, since current detection means for detecting the charging current at the low-level voltage is required, a current detection element is serially inserted in the charging circuit. Thus, there is a problem in that electric power loss occurs. Further, it is necessary to make the value of a current detecting resistance large so as to detect when the charging current is zero. Accordingly, there is another problem in that the electric power loss becomes greater, and at the same time, a complex circuit is required.
Further, generally, a secondary battery is used as a power source in mobile radio communication equipment such as a mobile phone. Especially, a lithium ion battery has a high energy density per unit area and per unit mass. Thus, it is possible to make equipment that includes a lithium ion battery smaller and lighter. When charging a lithium ion battery, the constant-voltage charge method that maintains a voltage of the battery to be constant, or the constant-current/constant-voltage charge method that performs constant-voltage charge after constant-current charge is employed. In a charging circuit, irrespective of the method applied thereto, charging is completed by detecting that the charging current is equal to or less than a predetermined full charge current during the constant-voltage charge.
In the following, a description will be given of a conventional charging circuit for a secondary battery. FIG. 4 is a diagram showing a conventional charging circuit for a secondary battery. In FIG. 4, the charging circuit includes an AC adapter 110, an adapter detection circuit 112 that detects that the AC adapter 110 is connected, a battery voltage detection circuit 116 that detects the voltage of a secondary battery 114 that is to be charged, a constant-voltage circuit 118 that performs constant-voltage charge on the secondary battery 114, a charge current detection circuit 122 that detects the charging current flowing to the secondary battery 114, a resistor R1 across which the charging current causes a voltage drop, a diode D1 that blocks a current from flowing from the secondary battery 114 to the AC adapter 110, and a charge control circuit 124 that performs drive control of the constant-voltage circuit 118. The AC adapter 110 is connected to a terminal 130. The constant-voltage circuit 118 includes a constant-voltage generation circuit 140 that generates a reference voltage BE1, a control transistor M1, and an operational amplifier A1. In addition, the charge current detection circuit 122 includes a constant-voltage generation circuit 142 that generates a reference voltage BE2 and an operational amplifier A2. Further, the adapter detection circuit 112 includes a constant-voltage generation circuit 144 that generates a reference voltage BE3 and an operational amplifier A3. The resistor R1 is connected between the AC adapter 110 and the control transistor M1. The diode D1 is connected between the control transistor M1 and the secondary battery 114.
In the following, a description will be given of the operation of this charging circuit. When the AC adapter 110 is connected to the charging circuit via the terminal 130, and the voltage of the AC adapter 110 is equal to or more than a predetermined value, the adapter detection circuit 112 outputs a predetermined signal Sg1 to the charge control circuit 124. In addition, the battery voltage detection circuit 116 detects the battery voltage of the secondary battery 114 and outputs a battery voltage signal Sg2. The charge control circuit 124 starts the operation when the signal Sg1 is input from the adapter detection circuit 112, and outputs a predetermined charge control signal Sg5 to the constant-voltage circuit 118. The constant-voltage circuit 118 starts the constant-voltage charge of the secondary battery 114 when the charge control signal Sg5 is input. While charging, the diode D1 prevents a current from flowing back to the AC adapter 110 from the secondary battery 114 via the control transistor M1 and the resistor R1. The charging current causes a voltage drop across the resistor R1, and the resulting voltage is applied to the charge current detection circuit 122. When the charge current detection circuit 122 detects from the input voltage that the charging current is lower than a predetermined value, the charge current detection circuit 122 sends a predetermined charge completion signal Sg6 to the charge control circuit 124. When the charge completion signal Sg6 is input to the charge control circuit 124, the charge control circuit 124 outputs the charge control signal Sg5 and stops the operation of the constant-voltage circuit 118.
As mentioned above, in order to detect the charging current, the conventional charging circuit uses the resistor R1. However, at the beginning of charging, the charging current is high and a sharp voltage drop results. Thus, heat generation of the resistor R1 becomes very high. In addition, power loss due to the heat generation is also great. In order to reduce such heat generation and waste of power, it is conceivable to make the resistance value of the resistor R1 small. However, by performing the constant-voltage charge, the current when charge complete is detected is small, and since the voltage drop across the resistor R1 is low, an input offset voltage of the operational amplifier Al detecting the generated voltage is not negligible. In other words, there is a problem in that the accuracy of detecting the charging current is deteriorated. Further, there is also a problem in that, since operational amplifiers having a small offset voltage are expensive, the manufacturing cost increases when using them
Furthermore, a problem occurs when a large current is supplied to the secondary battery at the beginning of charging in a case where the secondary battery is in an over-discharged state. Accordingly, it is impossible for such a charging circuit to charge the secondary battery that is in an over-discharged state.